1. Field of the Invention
The present invention relates to a narrow-band laser apparatus for narrowing the spectral band width of a laser beam.
2. Description of the Related Art
There is known a narrow-band laser apparatus, such as a narrow-band excimer laser, for narrowing the spectral band width of a laser beam with use of a band-narrowing element. The narrow-band laser apparatus uses a self-amplification type optical resonator in order to efficiently obtain a laser beam having a narrowed band. In the narrow-band laser apparatus using the self-amplification type optical resonator, a laser beam generated by a laser excitation unit within a laser tube is reflected by a high-reflectance mirror. The high-reflectance mirror is situated on one side of the laser tube along the optical axis. The reflected laser beam is made incident on another part of the laser excitation unit. Thus, the laser beam is amplified in the laser excitation unit, and the amplified beam is output from the other side of the laser tube along the optical axis. This type of narrow-band laser apparatus can increase the output of the laser beam with a narrowed band several times.
An example of the narrow-band laser apparatus using the self-amplification optical resonator is disclosed in "LASER KENKYU"("Review of Laser Engineering"), Vol. 17, No. 1, 1989, p. 51, FIG. 1. This apparatus is shown in the accompanying FIG. 1. The apparatus comprises a laser tube 1 which is hermetically sealed and contains a gas laser medium. A laser excitation unit 2 is provided within the laser tube 1. The laser excitation unit 2 includes a pair of main electrodes (a cathode and an anode) 3 which are arranged apart from each other and face each other. The main electrodes 3 are supplied with electric power from a high-voltage power source (not shown). When electric power is supplied, a glow discharge occurs between the main electrodes 3. The gas laser medium is excited by the glow discharge, and a laser beam L is generated in a direction perpendicular to the direction of the discharge.
First and second windows 4 are formed in both end faces of the laser tube 1 along the optical axis. A first etalon 5 and a first high-reflectance mirror 6 are arranged on a first side region of the laser tube 1, which faces the first window 4. A second etalon 7 and a grazing incidence type diffraction grating 8 are arranged on a second side region of the laser tube 1. A first restriction plate 9 having a slit 9a is located between the first etalon 5 and the first window 4, and a second restriction plate 11 having a slit 11a is situated between the second etalon 7 and the second window 4. The restriction plates 9 and 11 allow the passage of only that part of the laser beam L generated between the main electrodes 3, which has a desirable optical characteristic.
The diffraction grating 8 has an adjustable mirror 12. The angle of the mirror 12 is adjustable. The mirror 12 is situated to face the diffraction grating 8. The adjustable mirror 12 determines the direction in which the laser beam L (diffracted beam) is emitted from the diffraction grating 8. The laser beam L from the diffraction grating 8 is reflected by a second high-reflectance mirror 13 situated o the second side of the laser tube 1. The diameter of the reflected beam is enlarged by a beam expander 16 comprising a concave lens 14 and a convex lens 15. The expanded laser beam L is input to a part of the laser excitation unit 2, which is other than the part in which the beam L was generated. The beam L is amplified by the unit 2 and is output from the first window 4.
In the above structure, the discharge space between the electrodes 3 within the laser excitation unit 2 is divided into a space for generation of a laser beam and a space for amplification of the laser beam. In other words, the space for generating the laser beam L must be separated from the space for amplifying the laser beam L with a narrowed band, because of the arrangement of optical parts necessary for returning the laser beam L to the laser excitation unit 2. As a result, the discharge space within the laser excitation unit 2 must be increased, and accordingly the entire apparatus must be increased in size.
In addition, in the conventional apparatus, the laser beam L is expanded by the beam expander 16 in order to enhance the amplification effect of the laser beam L. Thus, the laser beam L is output through optical parts such as the concave lens 14 and convex lens 15 of the beam expander 16. When the laser beam is passed through the concave lens 14 and convex lens 15, the beam is absorbed or scattered by these optical parts. Thus, the loss of the laser beam L increases, and the output laser beam L contains scattered component.